1. Field of the Invention
The present invention relates to a zoom lens system, and, in particular, relates to an ultra-compact zoom lens system suitable for a digital camera.
2. Description of the Prior Art
In recent years, cellular phones with camera-function and PDAs (Personal Digital Assistant) have rapidly become popular. Accordingly, further miniaturization of a digital camera containing an imaging device therein and higher image quality are in demand. In order to provide higher image quality, an imaging device is required to attain an image size of approximately ⅓ inch (diagonal image height=3.0 mm) or larger.
Likewise, a zoom lens system which is designed for higher pixelation has naturally been required to be further miniaturized and to attain higher image quality. However, in conventional zoom lens systems which are mainly utilized in cellular phones with camera-function, the number of lens elements of a zoom lens system is about three or four, which is not sufficient to attain higher image quality. On the other hand, a zoom lens system which has commonly been used in a digital camera includes six to eight lens elements; and it is understood that such a number of lens elements (six to eight) cannot attain sufficient miniaturization of the zoom lens system.
As a miniaturized zoom lens system, a negative-lead type zoom lens system in which a lens group having a negative refracting power is positioned at the most object-side therein has generally been employed. In this regard, the following two types of the negative-lead type zoom lens system have been well known:    (i) a negative-lead type zoom lens systems of a two-lens-group arrangement, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), and a second lens group having a positive refractive power (hereinafter, a positive second lens group), in this order from the object; and    (ii) a negative-lead type zoom lens systems of a three-lens-group arrangement, i.e., a negative first lens group, a positive second lens group and a third lens group having a positive refractive power (hereinafter, a positive third lens group), in this order from the object.
In the negative-lead type zoom lens system of the two-lens-group arrangement, an attempt has generally been made to achieve further miniaturization of the zoom lens systems by increasing the number of lens elements in the positive second lens group. Due to this arrangement, the magnification increases and the traveling distance of the positive second lens group becomes shorter.
In the negative-lead type zoom lens system of the three-lens-group arrangement, the number of lens groups increases, and so does lens elements. Consequently, the traveling distance of each lens group can be made shorter; however, for the purpose of attaining further miniaturization of the zoom lens system, it becomes difficult to determine a balance among the increase in the number of both lens groups and lens elements, the overall length of the zoom lens system, and the increase in the diameter of the lens groups.
The present invention is materialized in a zoom lens system of the three-lens-group arrangement, i.e., a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object. Namely, the present invention is to provide an ultra-compact zoom lens system of the three-lens-group arrangement, which is constituted by a fewer number of lens elements, and can meet higher pixelation of an imaging device.